Effects of Dam Regulation on the Hydrological Alteration And
Total Page：16
File Type：pdf, Size：1020Kb
water Article Effects of Dam Regulation on the Hydrological Alteration and Morphological Evolution of the Volta River Delta Mawusi Amenuvor 1, Weilun Gao 1, Dongxue Li 1 and Dongdong Shao 1,2,* 1 State Key Laboratory of Water Environment Simulation & School of Environment, Beijing Normal University, Beijing 100875, China; m.amenuvor_kfi@yahoo.com (M.A.); [email protected] (W.G.); [email protected] (D.L.) 2 Tang Scholar, Beijing Normal University, Beijing 100875, China * Correspondence: [email protected]; Tel.: +86-10-5880-5053 Received: 16 January 2020; Accepted: 27 February 2020; Published: 28 February 2020 Abstract: The Volta River in West Africa is one of the most regulated rivers influenced by dams in the world, and the regulation has resulted in substantial impacts on the hydrological alteration and morphological evolution of the Volta River Delta. However, comprehensive analyses of the relevant effects are still lacking to date. In this study, inter-annual variations of river discharge and sediment load for pre- and post-Akosombo Dam periods (1936 to 2018) were analyzed through simple regression and Mann–Kendall (MK) trend analysis whereas the intra-annual variations were dictated by the non-uniformity and regulated coefficients. The shoreline changes were further evaluated using Landsat remote sensing images (1972 to 2018) to explore the effects of hydrological alteration on the morphological evolution of the Volta River Delta. Hydrological analyses show that the inter- and intra-annual variations are much higher in the pre-dam period, suggesting the substantial regulation of the Akosombo Dam on the Volta River. The dam regulation has more significant effects on the sediment load delivered to the delta than the river discharge, which decreased by 92.32% and 23.23%, respectively. Morphological analyses show that the progradation-erosion of the Volta River Delta constantly fluctuates within a relatively small range (maximum 0.5%) after the 1970s. The relationship between the variations of the delta area and sediment load implicates that a quasi-equilibrium state may have been established at the Volta River Delta, given the current sediment load. Our findings provide references for the future regulation and restoration of the Volta River Delta. Keywords: dam regulation; hydrological alteration; morphological evolution; Volta River Delta 1. Introduction Deltas host more than half a billion of the world’s population and one of the most productive ecosystems on Earth [1,2]. However, the world’s deltas are subject to continuous climate change and anthropogenic activities, such as sea-level rise, subsidence, and reduced sediment load, etc. [1,3–6]. Among these natural and anthropogenic stressors, dam regulations that lead to hydrological alteration have caused substantial impacts on the morphological evolution of the world’s deltas [7–11]. Understanding the potential effects of dam regulations on the affected deltas’ hydrology and morphology thus becomes a vital task in their protection and restoration [12,13]. The Volta River in West Africa is considered one of the most regulated rivers by dams worldwide [8,14–16]. The Volta River Delta was richly supplied with sediments from the Volta River [17]. However, the construction of the Akosombo Dam in 1964 has intercepted and reduced significantly fluvial sediment supply from the river with a decreasing rate of 90% [17,18]. The drastically reduced sediment supply has caused severe adverse effects on the river delta [19], e.g., coastal erosion [20]. Furthermore, Water 2020, 12, 646; doi:10.3390/w12030646 www.mdpi.com/journal/water Water 2020, 7, x FOR PEER REVIEW 2of12 Water 2020, 12, 646 2 of 12 drastically reduced sediment supply has caused severe adverse effects on the river delta [19], e.g., thecoastal reduction erosion in [ fluvial20]. Furthermore, sediment supply the reduction to the Volta in fluvial River Delta sediment has resulted supply into thethe regimeVolta R shiftiver ofDelta the Voltahas resulted River Delta in the from regime the river-dominated shift of the Volta delta River to wave-dominatedDelta from the river delta-dominated [20,21]. In additiondelta to wave to the- reductiondominated of delta the fluvial [20,21 sediment]. In addition input, theto the high reduction alongshore of sediment the fluvial transport sediment has alsoinput exacerbated, the high thealongshore erosion ofsediment the delta transport [20]. has also exacerbated the erosion of the delta [20]. Considering thethe extent extent of damof dam regulations regulations on the on Volta the RiverVolta DeltaRiver and Delta the economicand the economic and ecological and importanceecological ofimportance the area [20 of,22 ],the significant area [20 attention,22], significant has been givenattention to the has delta been [17,18 given,20,23 ,24to]. the However, delta apart[17,18 from,20,23 Ly,24 (1980)]. However, [18] who apart quantitatively from Ly (1980) analyzed [18 the] w sedimentho quantitatively retention analyzed by the Akosombo the sediment Dam andretention its impacts by the on Akosombo the shoreline Dam of and the its delta impact severals on decades the shoreline ago, little of the research delta several has been decades conducted ago, tolittle quantify research the has subsequent been conducted influences to quantify of the dam the subsequent regulations influence on the hydrologicals of the dam alterations regulations and on morphologicalthe hydrological evolution alterations of theand Volta morphological River Delta. evolution of the Volta River Delta. InIn thisthis study, study, the the hydrological hydrological alterations alterations were were analyzed analyzed using monthlyusing monthly and annual and river annual discharge river anddischarge sediment and loadsediment from load 1936 from to 2018. 1936 The to 2018 inter-annual. The inter variations-annual variations for pre- and for post-Akosombo pre- and post- DamAkosombo periods Dam were periods analyzed were using analyzed simple using regression simple andregression the Mann–Kendall and the Mann (MK)–Kendall trend (MK) analysis trend in Sectionanalysis 3.1 in .Sec Thetion intra-annual 3.1. The intra variations-annual variations represented represented by the non-uniformity by the non-uniformity coefficient coefficient (Cy) and ( theCy) r regulatedand the regulated coefficient coefficient (Cr) were (C calculated) were calcula for theted pre-Akosombo for the pre-Akosombo (1936–1964), (1936 the–1964), pre-Kpong the pre (1970-Kpong to 1982)(1970 andto 1982) the post-Kpong and the post (1983–2018)-Kpong Dam(1983 periods–2018) toDam examine periods the to impacts examine of dam the regulationimpacts of during dam diregulationfferent periods during in different Section periods3.2. Available in Section Landsat 3.2. A remotevailable sensing Landsat images remote of sensing the Volta images River of Delta the fromVolta 1972River to 2018Delta were from interpreted 1972 to 2018 to analyze were interpreted the associated to morphologicalanalyze the associated evolution morphological in Section 3.3. Theevolution paper in is concludedSection 3.3. with The apaper summary is concluded of the major with findingsa summary of the of the study. major findings of the study. 2. Methodology 2.1. Study SiteSite The VoltaVolta RiverRiver DeltaDelta isis thethe coastalcoastal plainplain ofof thethe VoltaVolta River,River, which which ranges ranges from from 5 5°25◦250' toto 66°20◦200' NN 2 and 00°4◦40' to 11°10◦10'0 E along the eastern coast of Ghana with aa totaltotal areaarea ofof approximatelyapproximately 45624562 kmkm2 inin 2 Ghana [22]] andand anan activeactive portionportion nearnear thethe riverriver mouthmouth ofof approximatelyapproximately 24302430 kmkm2 [[2525],], formingforming thethe interfaceinterface betweenbetween thethe VoltaVolta RiverRiver andand thethe AtlanticAtlantic OceanOcean (Figure(Figure1 1).). As As one one of of the the largest largest riverriver deltasdeltas inin WestWest Africa,Africa, thethe VoltaVolta RiverRiver DeltaDelta isis aa sanctuarysanctuary forfor aboutabout 80%80% ofof migratorymigratory birdsbirds thatthat stop-overstop-over inin Ghana,Ghana, andand hostshosts importantimportant speciesspecies inin itsits lagoonlagoon andand mangrovemangrove habitatshabitats [[2020].]. InIn additionaddition toto itsits ecologicalecological functions,functions, thethe delta is also of crucial socio-economicsocio-economic importance [26]] with diverse economic activities comprisingcomprising agriculture,agriculture, fishery,fishery, saltsalt harvesting,harvesting, and and tourismtourism [ 22[22].]. The Akosombo and Kpong D Damsams were built on the Volta Volta River in 1964 and 1982, 1982, respectively. respectively. TheThe twotwo dams dams generate generate about about 1060 1060 MW MW of hydropower,of hydropower providing, providing 80% of80% Ghana’s of Ghana electric’s electric power power needs. Theneeds. Akosombo The Akosombo Dam is D aboutam is 100 about km 10 upstream0 km upstream from the fro riverm the mouth riveralong mouth the along river the channel, river channel and the, Kpongand the Dam Kpong is 24 D kmam downstreamis 24 km downstream of Akosombo of alongAkosombo the river along channel. the river The channel reservoir. The capacity reservoir and areacapacity forthe and Akosombo area for the Dam Akosombo are 147,960 Dam million are 147,960 m3 and milli 8482.250on m3 and km 82482, whereas.250 km those2, whereas for the those Kpong for Damthe Kpong are 200 Dam million are 200 m3 andmillion 25.2 m km3 and2. 25.2 km2. Figure 1. The